The present invention relates to digital image processing, and more particularly to color transformations of the kind typically associated with white balancing.
Images are affected by image viewing conditions, e.g. by background and surrounding colors and illumination. However, humans can adapt to the viewing conditions, and it may be desirable to adjust the digital representation of the image in accordance with human adaptation. For example, if a scene is illuminated by a red lamp, the scene may acquire a reddish tint. An object that would reflect white light under white illumination may reflect reddish light under red illumination and may look reddish. However, humans adapt to the red illumination and may stop noticing the reddish tint. If the image is faithfully captured by a digital camera, the reddish tint would be captured, and would be reproduced when the image is displayed with a printer or a computer monitor. If the reproduced image is viewed under white illumination, the viewer would notice the reddish tint because the white illumination would not allow the viewer to adapt to the tint. Therefore, it may desirable to remove the reddish tint from the reproduced image so that the viewer's perception of the reproduced image would approach the viewer's perception of the original image.
Human adaptation can be modeled by a “white balancing” process which transforms image colors using a linear transformation chosen based on human adaptation to a white color. Suppose for example that the scene illumination or some other viewing conditions change a white color W to some color Wnew (Wnew would be reddish in the red-lamp example above). Supposedly, humans would adapt to the viewing conditions and would perceive the color Wnew as W. Therefore, a linear transformation is chosen that maps the color Wnew into W. Then another linear transformation may be performed that would change W into a color that would be perceived as W under the viewing conditions of the reproduced image.